At least 230 people were wounded in clashes between police and protesters in Pakistan's capital Islamabad, as a fortnight-long political impasse took a violent turn.

The violence, which began late Saturday and continued early Sunday, broke out after around 25,000 people marched from parliament to the prime minister's house, where some attempted to remove barricades around it with cranes, an AFP reporter at the scene said.

Police in Islamabad (AP)

Police responded with tear gas and rubber bullets.

The protesters, led by cricketer-turned-politician Imran Khan and Canadian cleric Tahir ul Qadri, had been camped outside parliament house since August 15 demanding Prime Minister Nawaz Sharif quit amid allegations of vote rigging.

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The crisis took on a new dimension earlier in the week after the government asked the powerful army to mediate, raising fears the military would use the situation to enact a "soft coup" and increase its dominance over civilian authorities.

"There are 1,600 to 2,000 trained terrorists. They have 200 women who are trained in the use of firearms and they have come with the intention of occupying state buildings," defence minister Khawaja Asif told AFP as the fighting broke out late Saturday.

Tahirul Qadri speaks to supporters in Islamabad (EPA)

"These are buildings that are symbols of the state," he said. "Their attempts are being resisted. And we will resist these with full force."

AFP's correspondent at the scene said protesters were carrying batons and sling-shots.

The injured were rushed to Islamabad's two main hospitals, and the number of casualties is expected to rise as clashes continue.

Wasim Khawaja, spokesman for the Pakistan Institute of Medical Sciences hospital in Islamabad, told AFP that 164 injured people have been taken to his hospital, while the Poly Clinic hospital said it had received 70 wounded so far.

"Condition of two of them was critical. There were more than 20 policemen among the injured," Khawaja said, with a second doctor confirming that figure.

Khawaja also said most of the victims had been injured by rubber bullets, and that that 17 of them were women.

Khurram Ghuman, a spokesman at Islamabad's Poly Clinic hospital, told AFP: "There are 20 women among them and most have been affected by tear gas, but they all are in stable condition."

Demonstrations have also erupted in parts of the eastern city of Lahore and the port city of Karachi.

Protesters with batons have burned tyres and blocked roads in Lahore, an AFP reporter said. Police used batons and fired tear gas to disperse the demonstrators there.

The protests remained peaceful in Karachi, however.

Khan and Qadri both claim that the 2013 elections which saw Prime Sharif sweep to power were massively rigged.

Local and and foreign observers said the polls were credible, and analysts believe the protests have been coordinated by the powerful army as a means of re-asserting its dominance over civilian authorities.

Paramilitary troops and soldiers standing guard to protect the PM's house as well as other sensitive installations have not yet been called into action, despite the violence Sunday.

Women and children have been prominent among both protest groups, and the rising number of casualties is set to further polarise the already embittered political atmosphere.

But government officials defended the use of force. "The demonstrators attacked first. They wanted to occupy the prime minister's house and stage a sit-in there," said Asif Kirmani, an aide of Sharif.

Qadri, however, said the government began the violence.

"The march is heading to its destination, we were peaceful but government began the bloodshed," he said.

Khan told Express News during the violence that he was inside a shipping container in front of the prime minister's house.

"We will continue our struggle against the government, till our last breath," Khan said, calling for demonstrations across Pakistan.

Qadri was in a bullet-proof car close to the scene, according television images.

The government earlier struck a defiant note, issuing a statement saying that Sharif would not be stepping down.

"There is no question of resignation or proceeding on leave by Prime Minister Muhammad Nawaz Sharif, nor any member from the government side has made such suggestion," the statement said.

Sharif had earlier dismissed the two-week old impasse as a "tiny storm" that would end soon.

Khan alleges the 2013 general election that swept Sharif to power in a landslide was rigged as part of a conspiracy involving the former chief justice and thousands of election commission workers.

Qadri has demanded wholesale changes to Pakistan's political system, and called for an interim "unity government" while they are implemented.

The leaders have drawn thousands to the streets of Islamabad, but their call has not mobilised mass support in a country of 180 million people.